1. Field
The present invention relates to an islanded power system with a distributed power supply.
2. Description of the Related Art
In recent years, a “micro grid” for reducing the burden on a commercial system through a load following operation of a distributed power supply has been actively developed. In an energy supply system using a distributed power supply adopting the concept of the micro grid (hereinafter, simply referred to as a micro grid), there is a demand for a load following operation as follows. In a normal state, a connected operation is performed in which an electric generation amount is controlled so that the quantity of purchased power from the commercial system becomes stable. In an emergency state such as a blackout, an islanded operation is performed in which high-quality power (having a small fluctuation in voltage and frequency) is supplied into the micro grid system.
When the convenience of a building is taken into consideration, in an emergency state such as a blackout, it is desirable to construct a system that may switch to the islanded operation from the connected operation while maintaining high-quality power supply, without causing momentary interruption. Accordingly, the building may be continuously managed without a momentary stop inside the micro grid system, such as a computer comparatively sensitive to the quality (a fluctuation in voltage and frequency) of power, being influenced by an external blackout.
As the related art of switching to the islanded operation from the connected operation without causing momentary interruption, there is known an uninterruptible power supply switchable to an islanded operation disclosed in Patent Documents 1 and 2. This system will be briefly described by referring to FIG. 3. This system includes two types of power supplies, that is, a rotating machine generator 6 and a fuel cell 7. For the communication of measurement data measured by measurement equipment 31, 41, 51, 61, and 71 or a control signal output from a control unit 1, for example, a high-speed signal line such as an analog signal line is used.
Furthermore, in FIG. 3, the bold solid line indicates a power line, and the thin solid line indicates a signal line (high speed).
During the islanded operation, the rotating machine generator 6 is controlled in accordance with a constant voltage control, so that a fluctuation in voltage inside the micro grid system is suppressed. In the case of the constant voltage control of the rotating machine generator 6, the voltage of the islanded power system may fluctuate when there is a high frequency load fluctuation between the loads 3, 4, and 5. For this reason, in this system, the high frequency load fluctuation is compensated by using the fuel cell 7, and only a low frequency load fluctuation is compensated in the rotating machine generator 6, so that fluctuations in voltage of supplied power are suppressed. Further, when the islanded operation is selected, a detection signal of the system abnormality detection unit 9 detecting abnormality of the commercial system 2 is transmitted to the breaker 8 via a high-speed signal line, and a control signal is supplied to respective power supplies (the rotating machine generator 6 and the fuel cell 7) via a high-speed signal line using the control unit 1. Accordingly, since the breaker 8 is opened and the control for the islanded operation of each power supply is selected at once, a momentary interruption does not occur.
However, in the system configuration shown in FIG. 3, since all signals are transmitted and received via the high-speed signal line, a problem arises in that cost increases due to the work of burying the signal line. In order to solve such a problem, as a modified example of the system configuration of FIG. 3, for example, a system configuration may be considered in which an existing low-speed signal line such as a LAN line is used. As shown in FIG. 4, a signal line enabling a high-speed signal transmission is used as only the signal line S1 between the system abnormality detection unit 9 and the breaker 8, and a low-speed signal line is used as the other signal lines, whereby an increase in cost due to the work of burying the signal line may be suppressed.
Furthermore, in FIG. 4, the bold solid line indicates a power line, the thin chain line indicates a signal line (low speed), and the thin solid line indicates a signal line (high speed).
However, in the uninterruptible power generating system switchable to the islanded operation (FIG. 3) shown in Patent Documents 1 and 2, when the islanded operation is selected, the operation mode of the fuel cell is changed from the constant power control to the constant voltage control, and hence only the system voltage is maintained. For this reason, there is a problem in that it is difficult to maintain the frequency of the micro grid system in the islanded operation. Further, both the rotating machine generator and the fuel cell as the power supplies have a minimum power generation output due to the principle of power generation. For this reason, when the sum of the load inside the micro grid system in the islanded operation is equal to or less than the sum of the minimum output of the generators, there is a problem in that the supply of power is stopped.
Further, as in the system configuration shown in FIG. 4, when a LAN line is used as an existing low-speed signal line, there is a time delay of an order of several seconds for transmitting and receiving data. For this reason, there are problems in that the fuel cell may not compensate a high frequency load fluctuation of an order of several seconds or less during the islanded operation due to the time delay caused by the low-speed signal line and the quality of supplied power is degraded. Further, when the islanded operation is selected, the load compensated by the rotating machine generator changes in a step shape (at a very high speed) for the time delay caused by the low-speed signal line in accordance with the power flow of the breaker, thereby causing a problem in that the quality of power is degraded.